Problem
Remarkable improvements in the yield and quality of cereal grains have been attained during the twentieth century by plant breeding and selection (Borlaug and Dowswell 1988). Nevertheless, such increases can not be sustained indefinitely. Of the many serious problems confronting humans, the most urgent is that of population growth. At the current rate of growth, world population is expected to double to 11 billion by the year 2030. This is already causing the loss of some of the best crop lands. To feed the world's population in 40 years' time, will require all our ingenuity. There is, therefore, an urgent need to increase food production by supplementing and complementing the traditional methods of plant improvement by the novel technologies of plant cell and molecular biology. These permit access to an unlimited gene pool by allowing the transfer of desirable genes between any two species of interest.
Plants are remarkable organisms. They are able to satisfy their energetical needs by means of the sunlight. In addition, they generate, by a process known as photosynthesis, carbohydrates from carbon dioxide. These substances can be used as energy stores, for biosynthesis as well as structural components. But, not all plant tissues are able to perform photosynthesis. All tissues which have no contact to light are non-photosynthetic (roots). Some tissues simply have other functions (developing seeds, vascular tissue). Naturally, photosynthesis can not be performed in the dark. During the light phase, photosynthetically active tissues must produce and store enough energy to supply the whole organism and if possible, to drive growth and reproduction. Usually, sugars are the means of short term storage (starch, sucrose) and transfer (sucrose) of energy. The main sinks for sucrose are developing leaves and non-photosynthetically active tissues. The non-photosynthetically active plant tissues must generate their complete organic components from the transferred carbon scaffolds (Dennis and Turpin 1990; Mohr and Schopfer 1978; Strasburger et al. 1991; Taiz and Zeiger 1991).
Respiration of the hexoses glucose and fructose derived from sucrose can generate up to 38 ATP* (Voet and Voet 1992). Simultaneously, carbon dioxide assimilated during FNT * Adenosine triphophate (ATP) is the energy currency of the cell. The change of free energy (.DELTA.G) for ATP hydrolysis is under physiological circumstances -50 to -65 kJ/mol. One NADH can be converted to 3 ATP. photosynthesis will be set free again. Photosynthesis needs 18 ATP and 12 NADPH to produce one hexose molecule (Mohr and Schopfer 1978; Strasburger et al. 1991; Taiz and Zeiger 1991). This equals an energy amount of 54 ATP. A simple calculation shows that 16 ATP are lost from photosynthesis to respiration.
Can respiration reduce crop yield? The answer is: Yes. A 20% reduction of respiration can lead to 10-20% increased crop productivity (Lambers 1985; Wilson and Jones 1982). Unluckily, in the field, one can not easily prolong light periods or optimize growth temperatures, air- and soil humidity. Therefore, other solutions to reduce loss of carbon dioxide by respiration must be developed.